Recently, with practical implementation of a blue light-emitting diode (LED), development of a white LED utilizing the blue LED is being aggressively pursued. A white LED ensures low power consumption and extended life compared with existing white light sources, and therefore, its application to liquid crystal panel backlight, indoor or outdoor lighting device, and the like is expanding.
The white LED developed at present is obtained by coating a Ce-doped YAG (yttrium.aluminum.garnet) on a surface of a blue LED. However, the Ce-doped YAG has a fluorescence wavelength in the vicinity of 530 nm and when the color of this fluorescence and the light of a blue LED are mixed to make white light, blue-tinted white light results and good white light cannot be obtained.
On the other hand, an α-sialon-based phosphor activated by a rare earth element is known to emit fluorescence with a longer wavelength than the fluorescence wavelength of Ce-doped YAG (see Patent Document 1). When a white LED is fabricated using fluorescence of this sialon, a white LED giving a bulb color at a lower color temperature than a white LED using YAG can be produced.
However, in order to meet the requirement for various white light sources, α-sialon that emits fluorescence in various colors enabling production of white color sources at various color temperatures has been sought. Therefore, a sialon-based phosphor capable of emitting fluorescence at a shorter wavelength is desired.
In this connection, as understood from Non-Patent Document 1, a Ca-containing α sialon phosphor is reduced in the fluorescence intensity when the fluorescence wavelength is shifted to the shorter wavelength side than 595 nm. Accordingly, it has been difficult to produce a sialon-based phosphor capable of emitting fluorescence at a short wavelength suitable for producing a high-luminance LED of daytime white color or daylight color by combining the phosphor with a blue LED.
To solve this problem, Patent Document 2 discloses a Li (lithium)-containing α-sialon-based phosphor. This sialon can emit fluorescence having a short wavelength compared with the Ca-containing α-sialon-based phosphor. Furthermore, this is a very convenient phosphor, because fluorescence having the same fluorescence wavelength as that of the Ca-containing α-sialon can be emitted and in turn, the range of color tones to which the phosphor can be applied is wide. In Patent Document 2, the phosphor is produced in a nitrogen-containing atmosphere under pressure, but production in an atmosphere under atmospheric pressure is commercially preferred in view of safety and cost of the apparatus. However, production in a nitrogen-containing atmosphere under atmospheric pressure has a problem that a phosphor with high fluorescence intensity cannot be easily produced. This is presumably attributable to the fact that the Li-containing α-sialon undergoes much evaporation of Li in the production process thereof and in turn, the crystal site of Eu as an element for light emission is unstable.